Amplifier comprising an electronic tube provided with collectors

ABSTRACT

An amplifier including an electronic tube with an axial electron beam, provided with a cathode and at least two collectors, and at least two sources of DC voltage. Each collector is connected to a DC voltage source having a potential difference such that, the further the collector is from the cathode, the lower the potential difference between this collector and the cathode. The DC voltage sources are connected together at a common point situated at the collector whose potential difference with the cathode is the lower but not zero.

The invention relates to an amplifier that comprises an electronic tube with axial electron beam and voltage sources; the tube comprises at least two collectors, each collector being connected to a voltage source.

The field of the invention is that of the power amplification of UHF signals, in particular the amplification of analog and digital television signals from terrestrial transmitters, by means of vacuum tubes comprising an axial electron beam.

It is briefly recalled that a vacuum tube uses the principle of the interaction between an electron beam and an electromagnetic wave in order to transmit to the wave a part of the power contained in the electron beam, so as to obtain at the output of the tube a wave of higher power than that of the wave injected at the input of the tube.

Several categories of vacuum tube exist. In traveling wave tubes, or TWTs, and klystrons, the electron beam is modulated in velocity. In inductive output tubes, or IOTs, it is modulated in density.

The principle of this interaction applied to an IOT tube, shown in FIG. 1, will be described.

It comprises an elongated vacuum envelope composed, in part, of several insulation ceramics 23 a, 23 b, 23 c, 23 d, with an electron gun at a first end emitting an electron beam 12 and, at a second end, a first collector 14 a and a second collector 14 b. The electron gun comprises a cathode 16 that emits the electrons and a grid 18 that controls the flux of electrons as a function of the voltage applied to it.

The signal to be amplified, of power Pe, is injected between the cathode 16 and the grid 18 and thus modulates the voltage on the grid. The electron beam 12 crossing the grid is then modulated in density by the grid and the electrons are emitted in the form of packets; the time interval between two packets is equal to the period of the signal. The beam 12, shown in the form of packets of electrons, is substantially cylindrical over almost the whole length of the tube between the cathode 16 and the collectors 14 a and 14 b. This cylindrical form is obtained thanks, on the one hand, to the shape of the cathode 16, of the anode 24 and of the grid 18 and, on the other, thanks to an axial magnetic field that keeps the electrons close to the axis 10 of the tube.

A packet of electrons coming from the grid is accelerated before entering a drift tube 20 then into an interaction gap situated between two interaction electrodes 22 a, 22 b; this interaction gap is connected to a primary output cavity 26 in which it generates an electric field. When another packet of electrons reaches this interaction gap, it meets this electric field which slows it down. During this slowing, the kinetic energy of the electrons is converted into electromagnetic or microwave energy, in other words into output power Ps that is directed toward the application, for example via a coaxial feed through an insulator 25.

For a television signal, the power efficiency is generally of the order of 20 to 40%: it represents the part of the power of the electron beam converted into power in the amplified signal.

The remaining power in the electron beam, after its passage through the primary output cavity 26, is subsequently dissipated in the collector. The electrons then bombard the walls of the collector and transform their kinetic energy into heat.

The electrons that reach the collector have very variable energy levels. In order to improve the power efficiency of these tubes, the collector is divided into two collectors 14 a and 14 b that are electrically isolated from one another; each of these collectors has a potential applied to it that corresponds to one of the energy levels of the electrons. The second collector 14 b has a lower potential applied to it than the first 14 a, with respect to the cathode 16, in order to slow down the electrons that impact this collector and thus reduce the energy lost in the form of heat. An efficiency up to three times higher than the efficiency of a conventional tube can thus be obtained. Tubes comprising more than two collectors have already been produced as have tubes comprising a repelling electrode at the back end of the collector, this electrode generally being connected to the cathode.

The collector 14 b is connected to the positive pole 28 of a DC voltage source 30, for example of 26 kV. The negative pole 32 of the DC voltage source 30 is connected to the cathode 16.

The collector 14 a is connected to the positive pole 34 of a DC voltage source 36, for example of 34 kV. The negative pole of the DC voltage source 36, common to that of the voltage source 30, is also connected to the cathode 16. The two sources therefore have a common point 32 situated at the cathode. The output cavity 26 is also connected to the positive pole 34 of the DC voltage source 36, if required via a measurement shunt between ground 17 and the casing of the tube that comprises the anode 24 and the two interaction electrodes; this measurement shunt allows the current intercepted by the body to be measured. The output cavity 26 is connected to ground.

A current I of a few Amps (for example 2.5 A) coming from the cathode 16 is obtained in the electron beam 12.

According to this configuration, two voltage sources 30 and 36 of over 26 kV are used, which are bulky and costly given the value of their voltage, the distances necessary for their isolation and also their respective powers.

Furthermore, the potential difference between the two collectors 14 a and 14 b, also called the depression voltage, must not exceed 12 kV in order not to damage the tube, especially the ceramic 23 a situated between the two collectors. However, in this type of voltage source, when one of them is rapidly interrupted by the action of the safety systems or turns on faster than the other during power-up, a differential voltage that can reach 34 kV is generated between the two collectors 14 a and 14 b causing, possibly irreversible, damage. The necessity for synchronizing these voltage sources makes this configuration very constraining.

One important goal of the invention is therefore to overcome these drawbacks by modifying the manner of supplying the collectors and the cathode.

In order to achieve this goal, one subject of the invention is an amplifier comprising an electronic tube with axial electron beam, provided with a cathode and at least two collectors, the amplifier also comprising at least two sources of DC voltage, each collector being connected to a DC voltage source having a potential difference such that, the further the collector is from the cathode, the lower the potential difference between this collector and the cathode, characterized in that the DC voltage sources are connected together at a common point situated at the collector whose potential difference with the cathode is the lower but not zero.

This configuration thus allows sources of lower voltage value than in the prior art to be used and the size and cost of the voltage sources to be minimized.

Moreover, it is no longer indispensable to synchronize the power-up and the shut-down operations of the voltage sources since in no case can the potential difference between the collectors exceed that of the voltage source of low value. This improves the reliability of this type of equipment and preserves the tube.

According to one feature of the invention, the voltage sources are variable.

Preferably, the voltage source (or sources), defining a depression voltage between two collectors, is a current/voltage-limited voltage source. The use of this current/voltage-limited voltage source is especially recommended for the amplification of signals of variable mean power as in the case of analog television. Another subject of the invention is a transmitter comprising such an amplifier.

Other features and advantages of the invention will become apparent upon reading the detailed description that follows, presented by way of nonlimiting example and with reference to the appended drawings, in which:

FIG. 1, already described above, shows schematically an amplifier with an inductive output electronic tube comprising two collectors and with voltage sources disposed according to the prior art;

FIG. 2 shows schematically an amplifier with an inductive output electronic tube comprising two collectors and with voltage sources disposed according to the invention;

FIG. 3 shows schematically an amplifier with an inductive output electronic tube comprising three collectors and with voltage sources disposed according to the invention.

From one figure to another, the same references are used to denote the same elements.

FIG. 2 shows an exemplary embodiment of an amplifier implementing the invention; it comprises an inductive output electronic tube with at least two collectors. It comprises two collectors 14 a and 14 b in the example in the figure. The invention may also be applied to traveling wave tubes or klystrons and, more generally, to any electronic tube with axial electron beam.

The collector 14 b is connected to the positive pole 28 of a DC voltage source 30, for example of 26 kV. The negative pole of the DC voltage source 30 is connected to the cathode 16.

The collector 14 a is connected to the positive pole 34 of a DC voltage source 36′, for example of 8 kV. The negative pole of this voltage source 36′ is connected to the collector 14 b; it is in series with the voltage source 30. The common point 32 of these two voltage sources 36′ and 30 is at the collector 14 b, in other words at the one that is the further from the cathode. The output cavity 26 is also connected to the positive pole 34 of the DC voltage source 36′, if necessary via a measurement shunt.

It can be seen that an overall voltage source for the collector 14 a of 26 kV+8 kV equaling 34 kV is indeed obtained. This configuration thus allows a voltage source of 26 kV and a voltage source of low value, for example 8 kV, to be used instead of the voltage sources of 34 kV and 26 kV used in the example shown in FIG. 1. The low value of the voltage source 36′ allows a compact model to be used which minimizes the size of new transmitters and makes the incorporation of the tube into existing transmitters possible at a lower cost.

In addition, the potential difference between the two collectors 14 a and 14 b, in other words the depression voltage, cannot exceed the voltage of this source 36′, here 8 kV, whereas it could reach 34 kV in the example in FIG. 1. Indeed, when the voltage source 30 is turned off, the depression voltage is that of the voltage source 36′, equal to 8 kV, and when the voltage source 36′ is turned off, the depression voltage is zero. Therefore, it is no longer indispensable to synchronize the power-up and shut-down operations of the voltage sources since in no case can the potential difference between the collectors exceed that of the source of low voltage value. This improves the reliability of this type of equipment and prolongs the life of the tube.

A current/voltage-limited voltage source is preferably used for this source of low voltage value 36′. It is recalled that a current/voltage-limited voltage source is a stabilized and bounded voltage source with both a voltage setpoint and a current setpoint: the voltage source delivers the highest possible voltage such that at least one of these setpoints is reached. Thus, depending on the maximum current allowed by the collector 14 a, a current/voltage-limited voltage source is used that prevents an over-dissipation of this collector. The use of this current/voltage-limited voltage source is especially recommended for the amplification of signals with variable mean power as in the case of analog television. The reason for this is that, in this case, the amplitude modulation of the analog signal carrier leads to a modulation of the current of the signal to be amplified which effects the current obtained in this collector 14 a.

In the case of a fold-back of the voltage source 36′, in other words in the case of a fold-back of the depression voltage, the voltage source 30 is preferably feedback controlled by a setpoint established by the cathode 16—ground 17 voltage measurement in order to keep this voltage constant.

In FIG. 3, an exemplary embodiment of the invention is shown in the case of a tube comprising a third collector 14 c in addition to the two collectors 14 a and 14 b of the preceding figure. The electrical configuration is the same as in the example in FIG. 2, with an additional DC voltage source 38, for example of 6 kV, whose positive pole is connected to the collector 14 c. The negative pole of this voltage source 38 is connected to the collector 14 b. This DC voltage source 38 is in series with the voltage source 30. The point 32, common to the three voltage sources 38, 36′ and 30, is situated at the collector 14 b, in other words at the one furthest from the cathode.

The overall voltage source for the collector 14 c is 26 kV+6 kV, equal to 32 kV. The potential difference between the two collectors 14 c and 14 b, in other words the depression voltage, cannot exceed the voltage of this source 38, here 6 kV. A current/voltage-limited voltage source can also be used for this source of low voltage value 38.

The invention may also be applied to frequency bands other than the UHF band, such as for example the following frequency bands: L (from 1 to 2 GHz), S (from 2 to 4 GHz), C (from 4 to 8 GHz), X (from 8 to 12.4 GHz), Ku (from 12.4 to 18 GHz), K (from 18 to 26.5 GHz), Ka (from 26.5 to 40 GHz), etc.

Another subject of the invention is a transmitter comprising such an amplifier. This is, for example, a terrestrial analog or digital television signal transmitter. 

1-7. (canceled) 8: An amplifier comprising: an electronic tube with an axial electron beam, provided with a cathode and at least two collectors; and at least two DC voltage sources, each collector being connected to a DC voltage source having a potential difference such that, the further the collector is from the cathode, the lower the potential difference between this collector and the cathode, wherein the DC voltage sources are connected together at a common point situated at the collector whose potential difference with the cathode is the lower but not zero. 9: The amplifier as claimed in claim 8, wherein the at least two voltage sources are variable. 10: The amplifier as claimed in claim 8, wherein one of the at least two voltage sources, defining a depression voltage between two collectors, is a current/voltage-limited voltage source. 11: The amplifier as claimed in claim 9, wherein one of the at least two voltage sources, defining a depression voltage between two collectors, is a current/voltage-limited voltage source. 12: The amplifier as claimed in claim 10, further comprising feedback control means for the voltage source situated between the common point and the cathode to keep the voltage source constant in a case of a fold-back of the depression voltage. 13: The amplifier as claimed in claim 11, further comprising feedback control means for the voltage source situated between the common point and the cathode to keep the voltage source constant in a case of a fold-back of the depression voltage. 14: The amplifier as claimed in claim 8, wherein the tube is configured to operate in the UHF or L or S or C or X or Ku or K or Ka band. 15: The amplifier as claimed in claim 8, wherein the tube is an inductive output tube or a traveling wave tube or a klystron. 16: A transmitter comprising an amplifier according to claim
 8. 17: A transmitter comprising an amplifier according to claim
 9. 